Cubic boron nitride (c-BN) films were deposited on Si wafers by ion-beam assisted deposition (IBAD). Both the structure of the film and the interface between the film and the substrate were investigated by cross-sectional transmission electron microscopy (TEM). Two types of specimens were observed. One was formed by IBAD which was performed by irradiating nitrogen with argon ions accelerated to an energy of 0.5 keV and evaporating boron simultaneously. The other was also formed by IBAD under the same conditions as above but employing ion beam dynamic mixing (IBDM), which was performed by irradiating the sample with ions accelerated to a rather high energy of 7 keV and evaporating boron simultaneously. The formation of c-BN was observed in both specimens by electron diffraction patterns and high resolution electron micrographs. The TEM observation clearly showed that the interface between the film and the substrate was different with or without IBDM. The interface of the speicimen formed with IBDM was diffuse, while that of the specimen formed without IBDM was sharp. The formation of the diffuse interface is thought to improve the adhesion of the film.